.page-404 {
    position: relative;
    height: 600px;
}

.container {
    height: 400px;
    width: 580px;
    position: absolute;
    top: 142px;
    left: 484px;
}

.container .btn a {
    position: absolute;
    top: 240px;
    left: 385px;
    background-color: #fff;
    height: 60px;
    width: 200px;
    text-align: center;
    line-height: 60px;
    transform: translate(-50%, -50%);
    font-size: 24px;
    color: #fff;
    text-transform: uppercase;
    text-decoration: none;
    font-family: sans-serif;
    background: linear-gradient(90deg, #03a9f4, #f441a5, #ffeb3b, #03a9f4);
    background-size: 400%;
    border-radius: 30px;
    z-index: 1;
}

@keyframes animate {
    0% {
        background-position: 0;
    }
    100% {
        background-position: 400%;
    }
}

.container .btn a:hover {
    animation: animate 8s linear infinite;
}

.container .btn a:before {
    content: '';
    position: absolute;
    top: -5px;
    bottom: -5px;
    right: -5px;
    left: -5px;
    z-index: -1;
    background: linear-gradient(90deg, #03a9f4, #f441a5, #ffeb3b, #03a9f4);
    background-size: 400%;
    border-radius: 40px;
    opacity: 0;
    transition: 0.5s;
}

.container .btn a:hover:before {
    filter: blur(20px);
    opacity: 1;
    animation: animate 8s linear infinite;
}